PROBLEM TO BE SOLVED: To provide method of maintaining nuclear reactor safety using water injection cooling, which allows for keeping a nuclear reactor in a stable condition for a long period of time even in a situation where external power and emergency power sources as well as the sea water cooling capability are lost.SOLUTION: Out of tens of instrument piping pipes 14 connected from a nuclear reactor 1 to instruments 13 installed outside a reactor containment vessel for measuring pressure, water level, flow rate, and the like in the nuclear reactor, multiple instrument piping pipes 14 not used for direct monitoring are each connected to a high pressure washer pump 16, from which fresh water is injected into the reactor 1 to cool the inside of the reactor 1.
